Neuropeptides Function in a Homeostatic Manner to Modulate Excitation-Inhibition Imbalance in C. elegans

Abstract

Neuropeptides play crucial roles in modulating neuronal networks, including changing intrinsic properties of neurons and synaptic efficacy. We previously reported a Caenorhabditis elegans mutant, acr-2(gf), that displays spontaneous convulsions as the result of a gain-of-function mutation in a neuronal nicotinic acetylcholine receptor subunit. The ACR-2 channel is expressed in the cholinergic motor neurons, and acr-2(gf) causes cholinergic overexcitation accompanied by reduced GABAergic inhibition in the locomotor circuit. Here we show that neuropeptides play a homeostatic role that compensates for this excitation-inhibition imbalance in the locomotor circuit. Loss of function in genes required for neuropeptide processing or release of dense core vesicles specifically modulate the convulsion frequency of acr-2(gf). The proprotein convertase EGL-3 is required in the cholinergic motor neurons to restrain convulsions. Electrophysiological recordings of neuromuscular junctions show that loss of egl-3 in acr-2(gf) causes a further reduction of GABAergic inhibition. We identify two neuropeptide encoding genes, flp-1 and flp-18, that together counteract the excitation-inhibition imbalance in acr-2(gf) mutants. We further find that acr-2(gf) causes an increased expression of flp-18 in the ventral cord cholinergic motor neurons and that overexpression of flp-18 reduces the convulsion of acr-2(gf) mutants. The effects of these peptides are in part mediated by two G-protein coupled receptors, NPR-1 and NPR-5. Our data suggest that the chronic overexcitation of the cholinergic motor neurons imposed by acr-2(gf) leads to an increased production of FMRFamide neuropeptides, which act to decrease the activity level of the locomotor circuit, thereby homeostatically modulating the excitation and inhibition imbalance.

Different Neuropeptides Are Expressed in Different Functional Subsets of Cholinergic Excitatory Motorneurons in the Nematode Ascaris suum

“…46 Flp-18 has been shown to work in a homeostatic manner to regulate excitation/inhibition balances in locomotion via two G-protein coupled receptors, NPR-1 and NPR-5. 47 Both AF9 and the flp-18 peptides were identified as ligands for NPR-1. 48–50 …”

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

“…Serotonin released from the NSM neurosecretory cells inhibits locomotion, and two different serotonin receptors expressed on largely nonoverlapping sets of neurons redundantly mediate this effect (Gürel et al, 2012: PMID 23023001). Neuropeptides encoded by FLP-18 modulate locomotion, and three different receptors, NPR-1, NPR-4, and NPR-5, expressed in different sets of cells, redundantly mediate these effects of FLP-18 (Stawicki et al, 2013: PMID 23658528). FLP-18 signaling is quite complex, because other behavioral effects of FLP-18 peptides are mediated non-redundantly by NPR-1, and for these effects FLP-18 and FLP-21 neuropeptides both seem to activate NPR-1 (Choi et al, 2013: PMID 23764289).…”

“…Such genetic studies have generated results that are similar in complexity to those from using signaling assays in cultured cells for receptor deorphanization. Thus it appears that in vivo , individual receptors can be activated by multiple peptides and individual peptides can act through multiple receptors (Choi et al, 2013: PMID 23764289; Stawicki et al, 2013: PMID 23658528).…”

Mechanosensation circuitry in Caenorhabditis elegans: A focus on gentle touch

“…Proprotein convertase is essential for protein precursor processing that releases C. elegans neuropeptides. The FMRFamide-like peptides (FLPs), FLP-1 and FLP-18 are involved in the acetylcholine/GABA regulatory pathway since flp-18;flp-1;acr-2(gf) mutants exhibit a more severe convulsive phenotype [50,68]. Biochemical and behavioral analyses uncovered that specific neuropeptide receptors (NPRs) termed NPR-1 and NPR-5 mediate FLP-18 signaling.…”

“…Cell specific expression in a mutant background uncovered that NPR-5 functioned in the muscle while NPR-1 likely mediated the locomotory circuit within neruons that transduce signals to muscle. Thus, these peptides function through their receptors in a homeostatic manner to regulate circuit activity [68].…”